According to Benjamin Barber's 2013 book, If Mayors Ruled the World, it is cities, not nation-states that truly lead the way on global change. Why cities? Well, that's where most of the people are. More than half of the world's population lives in urban areas, according to the UN, and that proportion will increase to 66 percent by 2050. In the US, more than 80 percent of the population already live in and around cities.

But that's only half the reason Barber says cities, not nations, are the real global leaders. It's because mayors, on the whole, exhibit non-partisan and pragmatic styles of governance. As New York Mayor Fiorello La Guardia said, "There is no Democratic or Republican way of fixing a sewer."

COP21 may be the proof to that political pudding.

Cities Are Already Leading the Charge on Renewables

On December 4, Paris Mayor Anne Hidalgo gathered 1,000 city and regional officers at Le Bourget to make a grand statement to the negotiators at COP21: "Cities are not waiting for [you] to give us the solution. We are moving ahead and making a solution possible."

To prove it, the cities joined together to establish the Paris City Hall Declaration, in which the mayors "declare solemnly that climate change is our common challenge and that advancing climate solutions is a shared responsibility, and a matter of rights, equality and social justice." The civic leaders also state that an "effective, global response to climate change presents one of the greatest economic opportunities to the 21st century, will protect public health, and strengthen sustainable development mindful of human rights and women empowerment."

Within the declaration, leaders pledge to "[a]dvance and exceed the expected goals of the 2015 Paris Agreement" and "[s]upport ambitious long-term climate goals such as a transition to 100 percent renewable energy in our communities, or an 80 percent greenhouse gas emissions reduction by 2050."

"Everywhere you look - banks, universities, cities and states - people are moving away from fossil fuels and towards a clean energy future," May Boeve, co-founder and executive director of 350.org, told Planet Experts. "Politicians can either accelerate the transition by setting a clear goal, like getting off fossil fuels by 2050, and clear ways to increase our ambition in the years ahead, or they can keep dragging their feet while the rest of us charge ahead. The climate movement has never been stronger."

C40: 'The Leadership of Cities Is Unequivocal'

On December 8, the C40 Cities Climate Leadership Group (C40) and Arup released a report entitled Potential for Climate Action. The report offers a roadmap on how to achieve sustainable progress.

"Our research shows that cities still have many valuable opportunities to scale up their actions and make even greater strides towards emissions reductions and climate resilience," said Arup Group Chairman Gregory Hodkinson. "As international climate negotiations continue, there is a fundamental role for business and civil society, as well as government at all levels, to facilitate cities' ongoing climate leadership."

According to research by C40 and Stockholm Environment Institute (SEI), the current trends of consumption and infrastructure development will "lock" the planet on course for more than 2°C temperature rise. That makes the next five years critical for reducing carbon emissions and transitioning to a clean energy economy. Already, C40 cities have taken 10,000 climate actions since COP15 in Copenhagen (2009), and they plan to do much more.

"The road from COP21 is now clear because we understand very concretely the further action cities can take to make a global impact in tackling climate change," said C40 Chair Mayor of Rio de Janeiro Eduardo Paes. "The leadership of cities is unequivocal, but there is no question that barriers remain - the most significant being access to finance and a need for greater governmental coordination."

Global mayors face two major challenges to climate action. C40 reports that 21 percent of the challenges identified by city leaders are related to resources and financing, which means limited budgets and "the perceived conflict between economic growth and climate action." This "perceived conflict" is often perpetuated by politicians with ties to conventional fossil fuels, despite the fact that economists have calculated that transitioning to sustainable energies could save cities a collective $17 trillion by 2050. Twenty percent of the challenges identified by cities relating to climate change involves difficulties in collaboration between the private sector and national governments.

Most importantly, in three out of four cases, says C40, these challenges cannot be overcome without the cooperation of the private sector, civil society and national and regional governments.

COP21 Is Not Going to Change Everyone's Mind Overnight

At the December 6 World Climate Summit, I spoke to Climate Reality President Ken Berlin, who opened a panel discussion on the economic importance of snow to alpine economies. Berlin noted that no matter what agreement resulted from the COP, climate deniers will not be swayed overnight. Especially not the ones whose personal fortunes are tied to the fossil fuel industry.

"I don't think it's going to change simply from what happens here," he said. "We had the first 11 Republican members of the House announce a couple months ago that they were going to support climate change. ...It will probably start with them backing renewable energy, which a lot of governors are doing. But for right now they're going to fight this until after the next election."

That puts the burden of climate action squarely on the shoulders of municipal leaders, who have thus far proven far more nimble and bipartisan when it comes to sustainable progress.

To this reporter, having seen so much and spoken to so many in so little a time, I can't think of a decent way to end this apart from sharing what Rasmenia Massoud said about cities in her novel, Broken Abroad: "A city isn't so unlike a person. They both have the marks to show they have many stories to tell. They see many faces. They tear things down and make new again."